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||||III USOO5485165A United States Patent (19) 11 Patent Number: 5,485,165 Foard 45 Date of Patent: Jan. 16, 1996

54 BROADBAND HIGH EFFICIENCY FULL 5,363,113 il/1994 Mametsa et al...... 343,744 WAVE OPEN COAXAL STUBLOOP OTHER PUBLICATIONS Investigation of Parasitic Loop Counterpoise Antenna, Sen 75) Inventor: Richard D. Foard, Culpeper, Va. gupta et al, IEE Transactions on Antenna and Propagation, vol. AP-17, No. 2, Mar. 1969 pp. 180-191. 73) Assignee: The United States of America as represented by the Secretary of the Primary Examiner-Donald T. Hajec Army, Washington, D.C. Assistant Examiner Tan Ho Attorney, Agent, or Firm-Anthony T. Lane 21 Appl. No.: 290,273 57 ABSTRACT 22 Fed: Aug. 15, 1994 A broadband, high-efficiency loop radiator is characterized Int. Cl...... H010 11/12 by a coaxial loop one in circumference with a 51 continuous center conductor fed through a coaxial T feed 52 U.S. Cl...... 343/741; 343/743; 343/842 and a discontinuity of the outer shield next to the feed point 58 Field of Search ...... 343/741, 743, of 50 ohms input impedance. Radiation occurs as a result of 343/742, 744, 745, 842, 866, 870, 815, this discontinuity resulting in a reflected 818, 819, 833, 834; 333/26 wave back down the shield. The one wavelength dimension of the loop generates two in phase electric dipoles spaced (56) References Cited one diameter apart resulting in a bidirectional radiation U.S. PATENT DOCUMENTS pattern of 3 dBill and a 30% operational bandwidth due to the frequency compensation of the open . The open stub 2,423,083 7/1947 Daubaras ...... 343.842 2,495,747 1/1950 Lundburg ...... 343,741 acts as a low loss parallel tuned circuit appear 2,615,134 10/1952 Carter...... 343/742 ing across a generator and the series impedance of the loop 3,902,177 8/1975 Mori et al...... 343/842 antenna resulting in increased bandwidth. 4,083,006 4/1978 Yakoshima ...... 34.3/842 4,983,985 l/1991 Beatty ...... 343,741 7 Claims, 8 Drawing Sheets

8 2 U.S. Patent Jan. 16, 1996 Sheet 1 of 8 5,485,165

22 STUB ANT 22 L - A R FIG. b :RA FG.2b

Gain (dB) THREE - LOOP YAG (Li+ L2+ L3)

TWO LOOP YAG: (Ll + L2)

DRIVEN LOOP (L)

38O 390 4OO 4O 42O 43O 44O 45O 46O 47O 53O FREQUENCY (MEGA HERTZ) FIG. O U.S. Patent Jan. 16, 1996 Sheet 2 of 8 5,485,165 HORIZONTAL REF DIPOLE +2.2dB;

U.S. Patent Jan. 16, 1996 Sheet 3 of 8 5,485,165 FG. 4d Sl SWR REF

START 3OO OOO OOO MHz 445 STOP 6OO OOOOOO MHz

FG. 4b

U.S. Patent Jan. 16, 1996 Sheet 4 of 8 5,485,165 F. G. 4C St SHR REF

START 3OOOOO 900 MHz STOP 6OOOOO 900 MHz

FG. 4d

U.S. Patent Jan. 16, 1996 Sheet S of 8 5,485,165

HORIZONTAL REF DPOLE+ 2.2dB GA = 3.2dB;

3OO9 o

29O. 7Oo

28Oo 8O

27Oo 900

26O OOo

25Oo |Oo

24O9 2Oo

F.G. 5 U.S. Patent Jan. 16, 1996 Sheet 6 of 8 5,485,165

VERTICAL REF DIPOLE + 2.2dB A-22dBit 7OdB = 9.2d Bi d P/2 BW = 8O (EL)

F. G. 6 U.S. Patent Jan. 16, 1996 Sheet 7 of 8 5,485,165

FG. 7 U.S. Patent Jan. 16, 1996 Sheet 8 of 8 5,485,165

FIG. 8 5,485,165 1. 2 BROADBAND HIGH EFFICIENCY FULL FIG. 1a is a plan view of the open stub full wave coaxial WAVE OPEN COAXAL STUBLOOP loop antenna according to the invention; ANTENNA FIG. 1b is a schematic representation of the dipole equiva lent for the antenna of FIG. 1a; BACKGROUND OF THE INVENTION 5 FIGS. 2a and 2b are diagrammatic and schematic repre With the advent of tuned loop antennas as a convenient sentations, respectively, of the circuit equivalent for the antenna structure for reduced operational space require antenna of FIG. 1a. ments, there is a need for a broadband loop antenna which FIG. 3a is a plan view of the current flow through the does not require tuning and yields near 100% efficiency. 10 antenna of FIG.1a and FIG.3b is a diagrammatic equivalent With conventional tuned loop structures, one to three dB of of the antenna of FIG. 3a; may be lost due to inherent losses of the LC FIGS. 4a and 4b are graphical representations of the networks operating at high current levels as a result of high VSWR and impedance, respectively, of the loop antenna of Q. This results in narrow operational bandwidths. FIG. 1a with a plane reflector; The present invention relates to single wire full wave 15 FIGS. 4c and 4d are graphical representations of the length loop antennas which are usually fed at a high imped VSWR and impedance, respectively, of the loop antenna of ance and which exhibit a narrow bandwidth. A full wave FIG. 1b without a reflector; open stub coaxial loop provides maximum efficiency and FIG. 5 is a graphical representation of the E-plane 3.2 dBi maximum bandwidth at a convenient inherent 50 ohm feed of the full wave loop antenna according to impedance. The electrical length of the coaxial loop is the invention; greater than 1.4 . FIG. 6 is a graphical representation of the E-plane radia BRIEF DESCRIPTION OF THE PRIOR ART tion pattern of the full wave loop antenna according to the invention arranged one quarter wavelength above a ground Prior loop antennas yield gains of -4 dBill (W4 loop over plane, a ground plane) at operating Q's of near 300. Efficient full 25 FIG. 7 is a graphical representation of the E-plane radia wave wire loop antennas without frequency compensation tion pattern of a two loop coaxial Yagi configuration with the yield Q’s of 20 or greater. longer reflector loop feed shorted and spaced at 0.2, The present invention was developed in order to provide FIG. 8 is a graphical representation of the E-plane radia the same high efficiency of prior loop antennas but with Q's 30 tion pattern of a three loop. Yagi configuration with the loops of 3-4 (25-30% bandwidth). This compares favorably with spaced at 0.2, the prior designs which have only about a 5% operational FIG. 9 is a graphical representation of the E-plane radia bandwidth. tion pattern of a folded dipole coaxial loop antenna; and SUMMARY OF THE INVENTION FIG. 10 is a graph showing the gain and bandwidth for the 35 open stub full wave coaxial loop antenna, and the two loop Accordingly, it is a primary object of the invention to Yagi, and three loop Yagi antennas. provide a broadband, high efficiency antenna radiator adapted for connection with an unbalanced and source. The antenna comprises a section of DETALED DESCRIPTION having a length of one wavelength. The cable has a center 40 The full wave open stub coaxial loop antenna 2 is shown conductor arranged in a continuous loop and an outer being in detail in FIG. 1a. It comprises a section of coaxial cable arranged in a shorter loop to define an open section. A 4, such as 50 ohm coaxial cable, having a length of one T-shaped connector connects the ends of the cable center wavelength . The cable includes a center conductor 6 conductor with the center conductor of the feed line and a arranged in a continuous loop and an outer shield 8 arranged bridging connector connects one end of the coaxial shield 45 in a shorter loop to define an open section or stub 10. with the shield of the feed line. The radiator is thus config The loop antenna 2 is connected with an unbalanced feed ured as a coaxial open stub loop which provides maximum line 12 and source (not shown). More particularly, a efficiency and bandwidth. The low Q or wide bandwidth of T-shaped connector 14 connects the center conductor 6 of operation is due to the equivalent parallel tuned circuit the loop with a center conductor 16 of the feed line and a achieved by use of the one wavelength open stub which 50 bridging connector 18 connects the shield 8 of the loop with comprises the loop itself. the shield 20 of the feed line. Due to the geometry of the According to another object of the invention, the loop antenna being in the form of a loop, the phase and Voltage configuration comprises a Yagi configuration for unidirec at the end of the center conductor 6 are the same as that of tional operation with greater gain. Two loop and three loop the feed line center conductor 16. Yagi configurations may be provided yielding high gain and 55 As will be developed in greater detail below, current extended operational bandwidths. flowing in the shield of the loop antenna generates two According to yet another object of the invention, the electric dipoles 22 shown in FIG. 1b. single loop may be narrowed physically for a wideband The principle of the design of the full wave open stub half-wave coaxial stub folded dipole having maximum gain 60 coaxial loop antenna is that of an unterminated, unbalanced and approximately 25% bandwidth. transmission line 24 which is near one and a half wave lengths long and provides an additional equivalent parallel BRIEF DESCRIPTION OF THE FIGURES tuned circuit in parallel with the series tuned circuit of the Other objects and advantages of the invention will loop with the generator 25 feeding the loop as shown in FIG. become apparent from a study of the following specification 65 2a. The circuit equivalent is shown in FIG. 2b. when viewed in the light of the accompanying drawing, in The open stub coaxial loop antenna operates at a lower Q which: or in a wider bandwidth due to the equivalent parallel tuned 5,485,165 3 4. circuit accomplished by use of the one wavelength open stub there is shown a coaxial loop in the form of a folded dipole which is actually the loop itself. The parallel tuned circuit 32 and its associated E-plane radiation pattern. The folded appears across the equivalent series tuned circuit of the loop dipole comprises a single loop narrowed physically to define with the generator in parallel. a wideband half-wave coaxial stub folded dipole with maxi The current flow through the open stub coaxial loop mum gain and 25% bandwidth. antenna 2 is shown in FIG. 3a with the diagrammatic In FIG. 10 there is shown a comparison of the gain equivalent being shown in FIG. 3b. The current flow in the achieved by the open stub coaxial loop (FIG. 5), the two shield of the loop generates two electric dipoles, one cen loop Yagi (FIG. 7), and the three loop Yagi (FIG. 8). As tered at the feed point and the other J4 or one diameter shown therein, the two and three loop Yagi arrays yield away across from the feedpoint of the loop antenna. 10 higher gains. The reflector and director elements of the Yagi Radiation occurs from the reflected standing waves from arrays are typically 12% longer and shorter, respectively, the unterminated end of the coaxial cable along the unter and the arrays yield 20 to 30% operational bandwidths. The minated shield of the cable. The of the driven element maintains its 50 ohm match without adjust coaxial cable does not slow the radiated wave since that ment with either or both a reflector or director added. radiation occurs on the outside of the shield as shown in FIG. 15 3a. This is verified by the physical length versus the elec With the full wave open coaxial stub loop antenna accord trical length of the coaxial cable determining the center of ing to the invention, there is provided a large gain bandwidth the operational frequency of the coaxial open stub loop device due to the frequency compensation of the open stub antenna. No or impedance transformer is required. and the elimination of the usual balun. 20 What is claimed is: The loop antenna according to the invention can be used 1. A broadband, high efficiency antenna radiator adapted with or without a ground plane reflector. FIGS. 4a and 4b. for connection with an unbalanced feed line and source, illustrate the VSWR and impedance of the antenna with a comprising ground plane reflector, while FIGS. 4c and 4d illustrate the VSWR and impedance of the antenna without a ground (a) a section of coaxial cable having a length of one plane reflector. 25 wavelength and including a center conductor and an In FIG. 5 is shown the radiation pattern generated by the outer shield, said center conductor being arranged in a two dipoles of the antenna. The dipoles are in phase Wapart. continuous loop and said outer shield being arranged in The radiation pattern has 3.2 dBill gain at bandwidths of near a shorter loop to define an open stub; and (b) means for connecting said cable with the feed line, 30%. The antenna configuration according to the invention 30 including a T-shaped connector arranged in said open results in a structure that is not easily detuned by a high stub for connecting the ends of said cable center dielectric loading environment. conductor with a center conductor of the feed line and Referring now to FIG. 6, there is shown in the center a a bridging connector for connecting one end of said broadband 50 ohm stub coaxial loop 2 placed over a 12 by coaxial shield with a shield of the feed line, thereby to 1, ground plane metal plate 26 at a height of 0.2. The 35 define a coaxial open stub loop which provides maxi E-plane radiation pattern is also shown. The positioning of mum efficiency and bandwidth. the antenna relative to the ground plane metal plate yields an 2. An antenna radiator as defined in claim 1, wherein said additional 6 dBigain for a total gain of 9.2 dBill with front coaxial open stub loop generates two equivalent in phase to back pattern ratios of 20 dB and cross polarization values dipoles, one centered at said connecting means and the other of-10 to -15 dB. The impedance and operational bandwidth 40 spaced diametrically opposed from said connecting means. of the loop radiator vary less than 10% with the additional 3. An antenna radiator as defined in claim 1, wherein said loading of the ground plane as compared to those shown in coaxial cable is configured as a two-element coaxial loop FIGS. 4b and 4d. Yagi. Loop arrays of up to four loops fed in phase and spaced 4. An antenna radiator as defined in claim 3, and further 0.8 apart (center to center) at a height of 0.2 above a 45 comprising a coaxial loop director. ground plane yield 15.2 dBill of forward gain, pattern front 5. An antenna radiator as defined in claim 1, wherein said to back ratios of near 20 dB, with E and H half power coaxial cable is configured as a three-element coaxial loop beamwidth of 32. Yagi. Other embodiments of the loop antenna and the E-plane 6. An antenna radiator as defined in claim 5, and further radiation patterns therefor are shown in FIGS. 7–9. More 50 comprising a coaxial loop director. particularly, in FIG. 7 there is shown a full-wave open stub 7. An antenna radiator as defined in claim 1, and further coaxial loop in the form of a two loop Yagi antenna 28 and comprising a ground plane reflector arranged parallel to said in FIG. 8 is shown a coaxial loop in the form of a three loop coaxial open stub loop and spaced from said loop by a Yagi 30. The two loop Yagi yields a 6.5 dBill forward gain distance of 0.2 wavelength. and the three loop Yagi yields a 7.2 dBill forward gain. Front 55 to back pattern ratios of 10–20 dB are achieved. In FIG. 9 ck k < 3